The imaging methods generally used for vascular diagnosis include both two-dimensional digital subtraction angiography (DSA) and three-dimensional rotational angiography. Both modalities have specific advantages in respect of diagnostic options. However both modalities also have specific disadvantages. They cannot simply be interchanged with one another.
Certain diagnosis processes require both temporal and structural information about blood flow. In such instances both technologies have to be deployed. However in order for the treating physician to be able to investigate certain facts in both data records under virtually identical conditions and particularly from the same viewpoint, the two-dimensional angiography images and the three-dimensional volume data record have to be registered in relation to one another.
Registration is often problematic, since a vascular tree that is permanently filled in its entirety with contrast agent is used for the reconstruction of the three-dimensional volume data record. The object of two-dimensional subtraction angiography in contrast is precisely the opposite, being to acquire the temporal propagation of a contrast agent in the vascular tree. Therefore the two-dimensional DSA sequence often contains no projection, which contains a vascular tree that is entirely (or at least essentially entirely) filled with contrast agent. This significantly impedes registration, as non-identical states have to be linked to one another. Add to this that in some instances during the recording of a 2D angiography image the patient moves and/or the vessels in the body are in constant (even if only slight) motion due to the pulse and blood pressure. Methods for compensating for such motion are known but compensation is difficult and only possible to a limited degree.
Registration methods per se are known from the prior art. Reference is made purely by way of example to the specialist article “Reconstruction of blood propagation in three-dimensional rotational X-ray angiography (3D-RA)” by H. Schmitt et al., which appeared in Computerized Medical Imaging and Graphics, vol. 29, pages 507 to 520, 2005.
To carry out registration it is known in the prior art that one of the two-dimensional images of an angiography sequence can be selected for example. In this process the angiography image showing the maximum degree of filling is preferably selected. Selection can be either automatic or manual. In this instance the selected DSA image is registered in relation to the volume data record. Because the recording geometry is generally kept constant during acquisition of the entire DSA sequence, registration is therefore valid for all the other images in the DSA sequence.
In other instances a “summation image” is determined based on all the projection images in the DSA sequence. For example the temporal gradient of the intensity of the individual images can be determined by pixel by pixel and each pixel, for which the temporal change deviates significantly from zero at least once, can be marked as being associated with the vascular tree. With this embodiment all the marked pixels correspond to the summation image, which is registered in relation to the volume data record.
Other methods are also known and possible. A method is thus described by way of example in the above-mentioned specialist article by H. Schmitt et al., wherein the entropy over time is determined for each pixel. The entropy codes the measure of random information in a system. It is defined as
      H    ⁡          (      x      )        =      -                  ∑                  z          ∈          Z                    ⁢                        p          ⁡                      (                          X              =              z                        )                          ⁢                  log          2                ⁢                  p          ⁡                      (                          X              =              z                        )                              p(X=z) here defines the probability that the pixel X will assume the color or gray-scale value z. Z is all the possible color or gray-scale values. In the context of the application this means that the intensity values of pixels associated with the vascular tree change markedly when considered over the entire sequence. The intensity change takes place precisely when the relevant part of the vascular tree has contrast agent flowing through it. The information content of the relevant pixel is defined by log2p(X=z). Either a very high or a very low value is assigned to the respective pixel depending on the entropy determined. It is possible in this manner to separate the vascular tree clearly from the background.
To determine the set of imaging parameters in the prior art according to the specialist article by H. Schmitt et al. the three-dimensional volume data record is not used directly. Instead an artificial projection of the volume data record is generated. Registration takes place between the two-dimensional summation image on the one hand and the artificial, likewise two-dimensional, projection of the volume data record on the other hand. However this procedure involves a significant computation outlay, as the artificial projection has to be determined as well as the determination of the summation image. Since the artificial projection is also the only component of the method that can be repeated as often as required, it must be calculated anew for every optimization run to determine the optimum set of imaging parameters.
In some instances DSA sequences are generated by means of so-called biplane x-ray systems. In this instance it is possible to acquire two projection images at two differing angulations, in other words with differing imaging parameters, simultaneously. This facilitates registration compared with a single projection image, since additional information is available from a second viewpoint. In many instances this additional information is sufficient to carry out a unique registration. The orientation of the two two-dimensional projection images is known through the parameters of the biplane x-ray system, so that point correspondences can be determined between the two images of the DSA sequences. However this technique also requires an artificial projection of the three-dimensional volume data record. This procedure is described in detail for example in the dissertation “Räumliche und zeitliche Rekonstruktion in der Neuroradiologie” (Spatial and temporal reconstruction in neuroradiology) by T. Hüllmandel, written in the year 2004 at Julius-Maximilian University in Würzburg.